Bulletin of the American Physical Society
18th Biennial Intl. Conference of the APS Topical Group on Shock Compression of Condensed Matter held in conjunction with the 24th Biennial Intl. Conference of the Intl. Association for the Advancement of High Pressure Science and Technology (AIRAPT)
Volume 58, Number 7
Sunday–Friday, July 7–12, 2013; Seattle, Washington
Session L6: CH.2 Chemistry: Metal Hydrides and Nitrides |
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Chair: Ranga Dias, Washington State University Room: Cascade II |
Tuesday, July 9, 2013 3:30PM - 4:00PM |
L6.00001: Building a Chemical Intuition Under Pressure: Prediction of Alkali Metal Polyhydrides and Subhydrides Invited Speaker: Eva Zurek Stabilization of solid phases with unusual combinations or stoichiometries, and unexpected electronic structures may be achieved by applying external pressure. The prediction of these structures using our chemical intuition (developed at 1 atmosphere) would be exceedingly difficult, making automated structure search techniques prudent. For this reason, we have written XtalOpt, an open-source evolutionary algorithm for crystal structure prediction. Whereas at 1 atmosphere the classic alkali hydrides combine in a one-to-one ratio, M$^+$H$^-$, under pressure non-classic stoichiometries MH$_n$($n > 1$) and M$_m$H ($m > 1$) are preferred. For example, theoretical work has predicted that LiH$_6$ and NaH$_9$ become particularly stable phases at about 100 and 25 GPa, respectively. And the potassium, rubidium and cesium polyhydrides all contain the H$_3^-$ anion, the simplest exaple of a three centered four electron bond. The alkaline-earth polyhydrides are considered as well. Chemical trends relating the stabilization pressure to the ionization potential, and the nature of the hydrogenic sublattice to the strength of the metal-hydride interaction can be made. These hydrogen-rich materials with nontraditional stoichiometries are computed to undergo an insulator to metal transition at pressures attainable in diamond anvil cells. It may be that these systems are superconductors at experimentally achievable pressures. The metal-rich region of the alkali/hydrogen phase diagram under pressure shows that alkali-metal subhydrides may also be stabilized under pressure. [Preview Abstract] |
Tuesday, July 9, 2013 4:00PM - 4:30PM |
L6.00002: Phase Transition of Rare-earth Metal Hydrides under High Pressure Invited Speaker: Duck Young Kim Hydrogen is the lightest and smallest element in the periodic table. Despite its most simple electronic structure, enormous complexity can arise when hydrogen participates in the formation of solids. High pressure perturbs the free energy sufficiently to push the system into unexplored regions of the energy landscape, thus providing an excellent platform for the investigation of novel physics in hydrides such as metal-insulator transition, superconductivity as well as stoichiometric change. In this talk, I will overview recent progress on hydrides research under pressure in both theoretical works and experiments. Theoretical predictions on atomic positions and stoichiometry in hydrides under high pressure play a critical role to determine crystal structures of experimentally observed novel compounds, especially due to tiny scattering length of hydrogen atoms in solids. In addition, predicted physical property such as metallization and superconductivity in hydrides can guide experiments and experimental observations provide inputs for refinement of calculations I will show examples to highlight the importance of integrated experiment-theory collaboration to study rare-earth hydride under high-pressure. [Preview Abstract] |
Tuesday, July 9, 2013 4:30PM - 4:45PM |
L6.00003: High-density Modifications in Hydrogen-Rich Compound Diborane Serge Desgreniers, Akio Yoshinaka, Yansun Yao, Dennis Klug The study of dense hydrogen-rich compounds is regarded as a way to investigate pathways to metallic hydrogen. Compression of hydrides may lead to a metallic state at lower pressures than that required for hydrogen. Condensed diborane represents an interesting hydride as its high dipole polarizability yields to a prediction of a metallic state below 100 GPa. And, at lower density, theoretical results indicate the possible formation of complex molecular crystal structures, as a function of compression, comprising not only dimers of BH$_{\mathrm{3}}$ (diborane) but also higher-order molecular assemblies, (BH$_{\mathrm{3}})_{n}$ with $n$\textgreater 2, and even polymeric chains. In this contribution, experimental results characterizing condensed phases of diborane, as obtained at high pressure at room temperature, are compared to predicted crystalline structures. Raman spectroscopy and x-ray diffraction with synchrotron radiation were carried out on single crystal as well as polycrystalline samples in diamond anvil cells for pressures up to 85 GPa. Results obtained at low pressures (\textless 4 GPa), across the liquid-solid phase boundary, indicate the existence of a phase with a crystalline structure different from that of the known b-phase ($P$2$_{\mathrm{1}}$/$n)$. Solid-to-solid transitions are observed at 6 and 14 GPa. The crystalline structures of the high-pressure phases, obtained by X-ray diffraction, are compared to molecular assemblies obtained theoretically. Finally, the possible metallization at high pressure is explored. [Preview Abstract] |
Tuesday, July 9, 2013 4:45PM - 5:00PM |
L6.00004: Disproportionation reaction of LaH$_2$ at high pressure and low temperature Akihiko Machida, Tetsu Watanuki, Daichi Kawana, Katsutoshi Aoki We have found that fcc-LaH$_2$ decomposes into two phases, which have different hydrogen compositions, a H-poor and H-rich phases, at 11 GPa at room temperature through synchrotron radiation x-ray diffraction (SR-XRD) measurements.\footnote{A. Machida \textit{et al.}, Phys. Rev. B \textbf{83}, 054103 (2011).} The decomposition proceeds spontaneously by pressurization, being interpreted in terms of a disproportionation reaction. Recent neutron diffraction measurements on LaD$_2$ confirmed the formation of a NaCl-type LaD as the D-poor phase.\footnote{A. Machida \textit{et al.}, Phys. Rev. Lett. \textbf{108}, 205501 (2011).} The disproportionation accompanies the transfer of H atoms from the tetrahedral to octahedral interstitial sites in the fcc metal lattice. The diffusivity of the H atoms in the metal lattice would be suppressed at low temperature. We hence investigated the pressure-induced disproportionation of LaH$_2$ at low temperature by SR-XRD measurements at BL22XU, SPring-8, and found the disproportionation occurred at 13.5 GPa even at 200 K. The volume fraction of the H-poor phase relative to the H-rich one at 200 K was larger than that of the H-poor phase at room temperature. The H-transfer by the disproportionation will be discussed in terms of temperature effect. [Preview Abstract] |
Tuesday, July 9, 2013 5:00PM - 5:15PM |
L6.00005: New routes to nitrogen-rich transition metal nitrides: Synthesis of novel polymorphs of Hf$_{3}$N$_{4}$ Ashkan Salamat, A.L. Hector, B.M. Gray, S.A.J. Kimber, P. Bouvier, P.F. McMillan One of the most obvious features of transition metal nitride chemistry is that the maximum formal oxidation state of the metal is rarely as high as in the corresponding oxides or fluorides. Much of the interest in the high oxidation phases stems from the desire to identify the next generation of photocatalytic materials with tuneable bandgaps. Experiments in the laser heated diamond anvil cell (LHDAC) between the direct reaction of metals and nitrogen have previously produced a number of important new main group nitride phases. This technique has also demonstrated its potential for formation of new nitrogen-rich transition metal nitride phases. Alternative methods with the development of ``soft'' routes to new phases with high nitrogen content also offer the possibility of obtaining metastable phases through topotactic conversions. Using LHDAC \textit{in situ} with synchrotron angle dispersive diffraction techniques we have crystallised at high pressures and temperatures two novel polymorphs of Hf$_{3}$N$_{4}$. Starting with an amide-derived nanocrystalline Hf$_{3}$N$_{4}$ sample we have identified a novel tetragonal ($I$4/$m)$ polymorph at 15 GPa and 1500K and a second high pressure orthorhombic (\textit{Pnma)} polymorph at 30 GPa and 2000 K. This study demonstrates that the combination of precursor-based synthesis and high-pressure crystallization could be very productive in synthesis of such nitrogen-rich phases. [Preview Abstract] |
Tuesday, July 9, 2013 5:15PM - 5:30PM |
L6.00006: Physical and chemical transformations of iron pentacarbonyl under pressure Young Yay Ryu, Choong Shik Yoo We have studied the physical and chemical transformations of iron pentacarbonyl (Fe(CO)$_{5})$ in externally-heated diamond anvil cells using \textit{in situ} micro-Raman and synchrotron x-ray diffraction. Raman spectra of Fe(CO)$_{5}$ are most characteristic to three different solid polymorphs and a polymeric solid found at high pressure-temperature condition, yielding the phase/chemical transformation diagram to 650 K and 20 GPa. The spectral results, for example, reveal that liquid Fe(CO)$_{5}$ undergoes several phase transformations to metastable solid phase I at 0.3 GPa, phase II at 1.5 GPa, and phase III at 4.8 GPa that polymerizes above 16 GPa. The X-ray diffraction data support the phase transitions that were observed in the Raman spectroscopy. These polymorphs also exhibit distinctive crystal morphology and optical properties, which will be discussed in this paper. [Preview Abstract] |
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